1. Field of the Invention
The present invention relates to a digital radio system, in particular for troposcatter and short-wave links, wherein, at the transmitting end, the digital useful signal is impressed in the form of a frequency modulation onto a radio-frequency carrier, and at the receiving end, following suitable demodulation of the received carrier, is available for further processing, and wherein the frequency diversity is employed in order to counter the transmission disturbances which are caused, in particular, by selective fading because of multiple path propagation.
2. Description of the Prior Art
Selective fading, caused by multiple path propagation, in particular, in the case of troposcatter and short-wave lengths, limit the availability of these lengths for the transmission of items of digital communications. As set forth, for example, in the German Pat. No. 26 28 997, corresponding to U.S. Pat. No. 4,197,501, which are fully incorporated herein by this reference, in the transmission of digital frequency-modulated data flows, especially when polydirectional antennae are used, under certain circumstances multiple path propagations lead to serious reception disturbances. Reflections from different directions mean that wave fronts emitted from the transmitting antennae strike the receiving antennae with different transit times. As a result of the vectorial addition of these wave fronts, the antennae base voltage experiences an amplitude response and phase response which are dependent both upon frequency and upon location. Because of the distortions and energy reductions (minima) which this energy distribution produces, for many frequencies and locations is causes a loss of the legibility of digital received signals. When the locating points of a transmitter and a receiver are predetermined, the frequency-dependent energy distribution results in a succession of relatively narrow energy minima and relatively wide energy maxima. The frequency spacing between two consecutive maxima or minima is referred to as the coherency band width of the radio link. In order to safeguard the useful signal transmission it is known, for example through the German allowed and published application No. 25 58 557 corresponding to U.S. Pat. No. 4,035,728, both of which are fully incorporated herein by this reference, to use so-called diversity measures. This consists in an exploitation of the fact that with different radio frequencies the described drop in level does not occur simultaneously (frequency diversity) and, therefore, parallel transmission at two or more frequencies increases the resistance to break downs. In addition to frequency diversity, there also exists so-called space diversity wherein the receiving antennae are constructed at an adequate distance from one another so that, as a result of different transit time conditions, these multiple path effects occur with a considerably lower degree of probability. Because of redundancy, diversity methods involve expenditures which are, in part, substantial.